(1) Field of the Invention
The present invention relates to a control valve for a variable displacement compressor, and more particularly to a control valve for a variable displacement compressor for controlling refrigerant displacement of a variable displacement compressor for an automotive air conditioner.
(2) Description of the Related Art
A compressor used in a refrigeration cycle of an automotive air conditioner is driven by an engine whose rotational speed varies depending on a traveling condition of the vehicle, and hence incapable of performing rotational speed control. To eliminate the inconvenience, a variable displacement compressor capable of changing the discharge amount of refrigerant is generally employed so as to obtain an adequate refrigerating capacity without being constrained by the rotational speed of the engine.
In a typical variable displacement compressor, a wobble plate is disposed within a crankcase formed gastight, such that the inclination angle thereof can be changed, and driven by the rotational motion of a rotational shaft, for performing wobbling motion, and pistons caused to perform reciprocating motion in a direction parallel to the rotational shaft by the wobbling motion of the wobble plate draw refrigerant from a suction chamber into associated cylinders, compress the refrigerant, and then discharge the same into a discharge chamber. In doing this, the inclination angle of the wobble plate can be varied by changing the pressure in the crankcase, whereby the stroke of the pistons is changed for changing the discharge amount of the refrigerant. The control valve for a variable displacement compressor provides control to change the pressure in the crankcase.
In general, the control valve for variably controlling the displacement of the compressor introduces part of refrigerant discharged at discharge pressure Pd from the discharge chamber into the crankcase formed gastight, and controls pressure Pc in the crankcase through control of the amount of refrigerant thus introduced. The amount of introduced refrigerant is controlled according to suction pressure Ps in the suction chamber. That is, the control valve for a variable displacement compressor senses the suction pressure Ps, and controls the flow rate of refrigerant introduced at discharge pressure Pd from the discharge chamber into the crankcase, so as to maintain the suction pressure Ps at a constant level.
To this end, the control valve for a variable displacement compressor is equipped with a diaphragm for sensing the suction pressure Ps, and a valve section for causing a passage leading from the discharge chamber to the crankcase to open and close according to the suction pressure Ps sensed by the diaphragm. Further, a type of the control valve for a variable displacement compressor which is capable of freely externally setting a value of suction pressure Ps to be assumed at the start of the variable displacement operation is equipped with a solenoid that enables configuration of settings of the diaphragm by external electric current.
By the way, conventional control valves for variable displacement compressors which can be externally controlled include a type for controlling a so-called clutchless variable displacement compressor that is configured such that an engine is directly connected to a rotational shaft on which a wobble plate is fitted, without providing an electromagnetic clutch between the engine and the rotational shaft for execution and inhibition of transmission of a driving force of the engine (see e.g. Japanese Unexamined Patent Publication (Kokai) No. 2000-110731 (Paragraph numbers [0010], [0044], and FIG. 1)).
This control valve comprises a valve section causing a passage leading from a discharge chamber to a crankcase to be opened and closed, a solenoid for generating an electromagnetic force causing the valve section to operate in the closing direction, and a diaphragm for causing the valve section to operate in the opening direction as suction pressure Ps becomes lower compared with atmospheric pressure, the valve section, the solenoid, and the diaphragm being arranged in this order. Therefore, when the solenoid is not energized, the valve section is fully open, whereby pressure Pc in the crankcase can be maintained at a level close to the discharge pressure Pd. This causes the wobble plate to become approximately at right angles to the rotational shaft, enabling the variable displacement compressor to operate with the minimum capacity. Thus, the refrigerant displacement can be substantially reduced to approximately zero even though the engine is directly connected to the rotational shaft, whereby the solenoid clutch can be dispensed with.
However, the conventional control valve for controlling a variable displacement compressor having no use for the electromagnetic clutch is configured such that the diaphragm and the valve section are arranged with the solenoid interposed therebetween, and the suction pressure Ps is introduced to the diaphragm which compares the suction pressure Ps with atmospheric pressure, via the solenoid. This necessitates the solenoid in its entirety to be accommodated within a pressure chamber, and hence components of the solenoid need to be designed with considerations given to resistance to pressure.
To eliminate this inconvenience, the present applicant has proposed a control valve for a variable displacement compressor configured such that the plunger of a solenoid is divided into a first plunger and a second plunger, and a diaphragm is interposed therebetween for sensing suction pressure Ps, whereby the valve lift of a valve section for controlling pressure in a crankcase is controlled by the second divisional plunger (Japanese Patent Application No. 2003-289581). Due to the arrangement described above, the diaphragm fluidically separates a space having the first plunger disposed therein and a space having the second plunger disposed therein from each other. Therefore, a section extending from the valve section to a portion where the diaphragm is disposed, including the second plunger which controls the valve lift of the valve section, is formed as a block to which pressure is applied, and the solenoid exclusive of the second plunger is not accommodated in the pressure chamber, allowing the same to be configured to be open to the atmosphere. Moreover, the second plunger which controls the valve lift of the valve section is urged in a direction away from the diaphragm, so that when the solenoid is not energized, displacement of the diaphragm is not transmitted to the valve section, and at the same time the valve section is held in its fully-open state, thereby enabling the variable displacement compressor to be controlled to the minimum displacement.
The first plunger and the second plunger as the divisional plungers of the solenoid are separated from each other when the solenoid is not energized, whereas when the solenoid is: energized, they are attracted to each other to behave as one plunger. Therefore, when the solenoid is energized, first, the first plunger and the second plunger are attracted to each other, and control is performed by these plungers integrated into one plunger in the same manner as conventionally performed.
However, in the control valve having a pressure-sensing section implemented by a diaphragm, the relative pressure between the suction pressure Ps and atmospheric pressure is sensed, and hence due to a change in atmospheric pressure between when the vehicle is running on a road at a high altitude and when the vehicle is running on a road at a low altitude, there occurs a control error.